1. Field of the Invention
The present invention relates generally to a multi-user Multiple-Input Multiple-Output (MIMO) communication system, and in particular, to a transmitting and receiving apparatus and method for reducing the complexity of transmit filter computation, for use in a Frequency Division Duplex (FDD) system and a Time Division Duplex (TDD) system.
2. Description of the Related Art
A multi-user MIMO communication system offers the benefit of increased spectral efficiency for a transmitter using multiple antennas. If a receiver also uses multiple antennas, the spectral efficiency is further increased because multiple links are established between the transmitter and a single user.
Further in the multi-user MIMO communication system, the same resources are accessible to a plurality of users, and independent data channels exist between the transmitter and a plurality of receivers. A multiplexing scheme called multi-user spatial multiplexing is used for multi-access.
For a higher spectral efficiency, complete channel information of the other communication party is required and non-linear processing should be performed for data transmission.
Accordingly, a Base Station (BS) should have knowledge of channel states of all Mobile Stations (MSs) for the downlink. However, it is generally difficult to acquire prior knowledge of the channel states in real implementation.
In a TDD system using a slow-varying fading channel, the BS can rely on channel reciprocity to acquire channel information of the MSs. Channel reciprocity means that downlink channel information can be derived from uplink channel information on the assumption that an uplink channel and a following downlink channel have the same channel characteristics in the TDD system using the same frequency band for transmission and reception.
However, if the assumption is not valid, the performance of a closed-loop multi-user MIMO scheme fails to approach the capacity of a MIMO broadcast channel.
Conventionally, the multi-user MIMO communication system is implemented by deriving downlink channel information of an MS based on channel reciprocity and sending a transmit filter to the MS through downlink sounding. Transmit filters are used to achieve spatial multiplexing of multiple users on the same time-frequency resources.
This conventional multi-user MIMO communication system is confined to the TDD system. It is not viable for an FDD system because channel reciprocity is not available. Since the transmit filter is computed using an iterative algorithm, computation complexity is increased. If a smaller number of computation iterations are performed to decrease the complexity, the accuracy of the transmit filter is decreased, thus leading to performance degradation.
Accordingly, there exists a need for an apparatus and method for overcoming the complexity problem encountered with filter computation, for use in the FDD system and the TDD system.